The present invention relates to a technology of diagnosing a failure of an exhaust purifying apparatus for purifying an exhaust gas discharged from an internal combustion engine.
An exhaust system of an internal combustion engine mounted in an automobile etc is provided with a catalyst device which retains a precious metal such as platinum, palladium etc serving as a catalyst for the purpose of purifying harmful gas components such as, e.g., carbon monoxide (CO), nitrogen oxide (Nox) or hydro carbon (HC) etc in an exhaust gas.
The catalyst device oxidates HC and CO in the exhaust gas into H2O and CO2 by making HC and CO react to oxygen O2, and simultaneously reduces Nox in the exhaust gas into H2O, CO2 and N2 by making Nox react to HC and CO in the exhaust gas.
Incidentally, a starting property is enhanced by setting an air fuel ratio of the engine lower than a theoretical air fuel ratio (a rich atmosphere) when starting the internal combustion engine, and hence a comparatively large amount of unburned gas component such as unburned hydro carbon (HC) is discharged. In particular, when the internal combustion engine is cold-started, a temperature of the internal combustion engine is low, and burning of a mixed gas becomes unstable. Hence, a large quantity of unburned gas component is to be discharged. The catalyst device described above is activated at a temperature above a predetermined temperature and is therefore brought into an non-activated state when lower than the predetermined temperature as in the case of the internal combustion engine being cold-started, with the result that the enormous amount of unburned gas component contained in the exhaust gas can not all purified. What is known as an apparatus for obviating the problem described above is an exhaust purifying apparatus constructed such that an adsorbing material for adsorbing the unburned gas component in the exhaust gas when lower than the predetermined temperature and desorbing the unburned gas component adsorbed thereto when over the predetermined temperature, is provided in an exhaust system disposed upstream of the catalyst device.
The exhaust purifying apparatus described above is intended to purify through the catalyst device the unburned gas component desorbed from the adsorbing material and the unburned gas component in the exhaust gas after the catalyst device has been activated while making the adsorbing material adsorb the unburned gas component in the exhaust gas when the catalyst device is in the non-activated state.
According to the above-described exhaust purifying apparatus, if an adsorbing performance of the adsorbing material declines due to a failure and deterioration etc, the unburned gas component contained in the exhaust gas can not be adsorbed to the adsorbing material but may be released into the atmospheric air. It is therefore of much importance to detect the failure and deterioration of the adsorbing material at a high accuracy and prevent the gas emission from worsening due to the failure and deterioration of the adsorbing material.
In response to such a request, there is proposed a known exhaust purifying apparatus disclosed in Japanese Patent Application Publication No. 8-93458. This exhaust purifying apparatus is constructed to judge the failure of the absorbing material on the basis of a temperature rising velocity in an adsorbing process of making the adsorbing material adsorb the unburned gas component, or a temperature rising velocity of the adsorbing material in a desorbing process of desorbing the unburned gas component desorbed to the adsorbing material.
An adsorption heat is evolved when the unburned gas component is adsorbed to the adsorbing material, and the temperature of the adsorbing material is increased by this adsorption heat. If the unburned gas component is adsorbed to an adsorbing material composed of porous activated carbon etc, however, interaction between the unburned gas component and the adsorbing material is weak, resulting in a physical adsorption in which no special chemical binding occurs. Hence, there must be a minute quantity of occurrence of the adsorption heat.
Therefore, according to the exhaust purifying apparatus, it is difficult to discern a change in temperature of the adsorbing material, which might cause a misguided diagnosis. Then, if the absorbing material is mis-diagnosed to be normal in spite of the adsorbing material having fallen into a failure, the gas emission worsens. Then, a temperature sensor exhibiting a high accuracy is required for preventing the mis-diagnosis described above.